The present invention relates to a scanning probe microscope for examining an electrical characteristic of a sample surface.
A Kelvin probe microscope and a scanning Maxwell microscope are scanning probe microscopes for examining an electrical potential distribution of a sample surface. These microscopes are already commercially available. Both microscopes have an extremely similar structure. First, the structure of the Kelvin probe microscope will be explained. Then, the structure of the scanning Maxwell microscope explained in view of its differences from the Kelvin probe microscope.
The schematic structure of the Kelvin probe microscope is shown in FIG. 3. A cantilever 306 has a probe 308 on its free end. The cantilever 306 is supported by a piezoelectric element 330. The piezoelectric element 330 oscillates upon reception of application of an AC voltage from an AC voltage supply unit 332. Then, the cantilever 306 is oscillated by the oscillation of the piezoelectric element 330. A sample 302 to be examined is mounted on a sample table 304. The sample table 304 is fixed to the free end of a tube scanner 310. A Z controller 312 controls the position of the tube scanner 310 in a Z direction of its free end to maintain a distance between the center of the oscillation of the top end of the probe and a sample surface to be constant. An XY scanning circuit 314 controls the position of the tube scanner 310 in an XY direction of its free end such that the probe is scanned over the sample surface.
The sample 302 is of metal or semiconductor, and the sample table 304 is formed of electrical conductive material. Then, both are electrically connected to each other. Also, the cantilever 306 and the probe 308 are made of conductive material. An AC voltage is applied between the cantilever 306 and the sample table 304 by a variable DC voltage supply unit 316 and an AC voltage supply unit 318. The AC voltage generates an electrical charge distribution changing periodically, in which a polarity to the top end of the probe and a polarity to the sample surface are opposite to each other. The electrical charge distribution generates electrostatic force, which periodically changes. The electrostatic force oscillates the cantilever 306. In other words, the cantilever 306 oscillates upon reception of mechanical force generated by the piezoelectric element 330 and electrical force generated between the probe and the sample.
A displacement meter 320 outputs a signal (displacement signal) showing the displacement of the free end of the cantilever 306. A preamplifier 322 amplifies the displacement signal from the displacement meter 320, and outputs the amplified signal to an AM demodulator 323. The AM demodulator 323 demodulates the output signal from the preamplifier 322 by the angular frequency of the AC voltage supply unit 332. In other words, the AM demodulator 323 extracts a component concerning to the angular frequency equal to that of the AC voltage supply unit 332 from the displacement signal. Then, the AM demodulator 323 outputs the extracted component to a lowpass filter 325 and a synchronism detector 326.
The lowpass filter 325 extracts a DC component contained in the output signal from the AM demodulator 323, and outputs the DC component to the Z controller 312. The Z controller 312 controls the displacement of the tube scanner 310 in its Z direction based on the signal from the lowpass filter 325. The output signal from the Z controller 312 is fetched to a data processing unit 336 as configuration data of the sample surface.
The synchronism detector 326 extracts a component concerning to the angular frequency equal to that of the AC voltage supply unit 318 from the output signal output from the AM demodulator 323. Then, the synchronism detector 326 outputs the extracted component to a voltage control circuit 328. The voltage control circuit 328 controls the voltage value of the variable DC voltage supply unit 316 based on the signal from the synchronism detector 326. The output signal from the voltage control circuit 328 is fetched to the data processing unit 336 as surface potential data of the sample.
The data processing unit 336 maps configuration data, which is output from the Z controller 312, and surface potential data, which is output from the voltage control circuit 328, referring to XY data from the XY scanning circuit 314. Thereby, a configuration image of the sample surface and a potential distribution image can be obtained.
The following will specifically explain the operation of this device.
The cantilever 306 oscillates upon reception of force F.sub.osc generated by the oscillation of the piezoelectric element 330, electrostatic force F.sub.es acting on the probe 308, and van der Waals force F.sub.vdw acting on the probe 308 receives. Force F.sub.osc is fixed regardless of the distance between the center of the oscillation of the top end of the probe and the sample surface. Each of F.sub.es and F.sub.vdw changes by depending on the distance between the center of the oscillation of the top end of the probe and the sample surface. Due to this, the oscillation of the cantilever 306 changes depending on the distance between the center of the oscillation of the top end of the probe and the sample surface.
The Z controller 312 controls the displacement of the tube scanner 310 in its Z direction to maintain the distance between the center of the oscillation of the top end of the probe and the sample surface to be constant. The change of the oscillation of the cantilever 306 is data, which is necessary for the above-mentioned control, and force f.sub.osc, which has no influence on the above-mentioned control, may be omitted from the consideration. In other words, force F, which the cantilever receives, may be set to F=F.sub.es +F.sub.vdw. The following explanation is based on the assumption of F=F.sub.es +F.sub.vdw.
It is assumed that the voltage of variable DC voltage supply unit 316 is V.sub.DC, that of AC voltage supply unit 318 is V.sub.e sin.omega..sub.e t, and that of AC voltage supply unit 332 is V.sub.m sin.omega..sub.m t. As compared with resonance frequency f.sub.0 of cantilever 306, the angular frequency .omega..sub.m of the voltage of AC voltage supply unit 332 is preferably set to .omega..sub.m =2.pi.f.sub.0 to oscillate the cantilever 306 at large amplitude.
Force F, which the cantilever 306 receives, can be expressed by the following equation (1) where a voltage between the probe 308 and sample 302 is set to V. ##EQU1##
In this case, a first term is electrostatic force F.sub.es. In the voltage V can be expressed by V=V.sub.S +V.sub.DC +V.sub.e sin.omega..sub.e t where a potential of the sample 302 is set to V.sub.S. Then, this equation is substituted for the equation (1), and the following equation (2) can be obtained: ##EQU2##
The displacement signal, which is output from the AM demodulator 323, corresponds to the equation (2). The synchronism detector 326 extracts a .omega..sub.e component, that is, a coefficient component of sin.omega..sub.e t of the equation (2) from the displacement signal, which is output from the AM demodulator 323. The voltage control circuit 328 controls the voltage V.sub.DC of variable DC voltage supply unit 316 such that the output signal of the synchronism detector 326 is maintained to be 0, that is, V.sub.S +V.sub.DC =0. Therefore, the control signal, which is output from the voltage control circuit 328, corresponds to the surface potential of the sample 302 so as to be fetched to the data processing unit 336 as surface potential data.
By the above control, the term containing V.sub.S +V.sub.DC disappears from the equation (2). As a result, force F, which the cantilever 306 receives, is expressed by the following equation (3), and the corresponding displacement signal is output from the displacement meter 320. ##EQU3##
The lowpass filter 325 extracts DC components, that is, components of first and second terms of the equation (3), from the displacement signal, which is output from the AM demodulator 323. Since the component of the first term is greatly larger than that of the second term, the signal from the lowpass filter 325 is dominantly influenced by the first term. In other words, the signal from the lowpass filter 325 substantially corresponds to the first term without reflecting the influence of the second term.
The Z controller 312 controls the displacement of the tube scanner 310 in its Z direction to maintain the output of the lowpass filter 325 to be constant. As a result, the control signal, which is output from the Z controller 312, corresponds to the configurations of the sample 302 so as to be fetched to the data processing unit 336 as configuration data.
The data processing unit 336 maps configuration data and surface potential data, referring to XY data from the XY scanning circuit 314. Thereby, a configuration image of the sample surface and a potential distribution image can be obtained.
Next, the following will explain the scanning Maxwell microscope. FIG. 4 shows the schematic structure of the scanning Maxwell microscope. The structure of the scanning Maxwell microscope is greatly similar to that of the Kelvin probe microscope, and an input signal source to the Z controller 312, that is, a synchronism detector 334 is slightly different from the structure of Kelvin probe microscope.
The synchronism detector 334 extracts a component of 2.omega..sub.e, that is, a coefficient component of cos2.omega..sub.e t of the equation (3) from the displacement signal, which is output from the AM demodulator 323. The Z controller 312 controls the displacement of the tube scanner 310 in its Z direction so as to maintain the output of the synchronism detector 334 to be constant.
The data processing unit 336 fetches the control signal, which is output from the voltage control circuit 328, as surface potential data. The data processing unit 336 also fetches the control signal, which is output from the Z controller 312, as configuration data. The data processing unit 336 maps configuration data and surface potential data, referring to XY data from the XY scanning circuit 314. Thereby, a configuration image of the sample surface and a potential distribution image can be obtained.
The Kelvin probe microscope obtains configuration data based on substantially the component of the first term of the equation (3). The scanning Maxwell microscope obtains configuration data based on the component of the coefficient of the third term, cos2.omega..sub.e t of the equation (3). These components of the first and third terms in the equation (3) are the same, except for the difference in the negative sign of cos2.omega..sub.e t. In both Kelvin probe microscope and the scanning Maxwell microscope, configuration data can be obtained by maintaining these components to be constant. In other words, configuration data can be obtained by maintaining a capacitance gradient (partial differential by Z of capacitance C) to be constant.
However, the capacitance between the probe and the sample is not always constant by depending on the locations of the sample surface. Due to this, configuration data, which is obtained under control of the constant capacitance gradient, does not correctly reflect the shape of the sample surface.
For example, in the case of the sample in which the equal conductor is exposed onto the surface, capacitance between the probe and the sample equally correspond to the distance between the top end of the probe and the sample surface. However, in the case of the sample having an insulating film on at least a part of the surface, the capacitance between the probe and the sample does not equally correspond to the distance between the top end of the probe and the sample surface. The capacitance changes depending on a thickness of an insulating film (a film thickness is considered to be 0 at a portion having no insulating film). Due to this, the capacitance gradient naturally changes depending on the thickness of the insulating film. Therefore, in the case of the sample having the insulating film on at least a part of the surface, the distance between the top end of the probe and the sample surface is controlled such that the capacitance gradient is maintained to be constant. Under such control, the distance between the top end of the probe and the sample surface is not always maintained to be constant. As a result, configuration data, which is obtained by maintaining the capacitance gradient to be constant, does not correctly reflect the shape of the sample surface.